NASA Technical Reports Server (NTRS) 20110006347: Growth and Characteristics of Bulk Single Crystals Grown from Solution on Earth and in Microgravity

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by  NASA Technical Reports Server (NTRS)

Growth and Characteristics of Bulk Single Crystals Grown from
Solution on Earth and in Microgravity
M. D. Aggarwal+, A. K. Batra, R. B. Lal
Department of Physics, P.O. Box 1268
Alabama A&M University
Normal, AL 35762, USA

Benjamin G. Penn and Donald O. Frazier
NASA / Marshall Space Flight Center, Huntsville, AL 35812, USA
ABSTRACT
The growth of crystals has been of interest to physicists and engineers for a long time
because of their unique properties. Single crystals are utilized in such diverse applications as
pharmaceuticals, computers, infrared detectors, frequency measurements, piezoelectric
devices, a variety of high technology devices and sensors. Solution crystal growth is one of
the important techniques to grow a variety of crystals when the material decomposes at the
melting point and a suitable solvent is available to make a saturated solution at a desired
temperature. In this chapter an attempt is made to give some fundamentals of growing
crystals from solution including improved designs of various crystallizers. Since the same
solution crystal growth technique could not be used in microgravity, authors had proposed a
new cooled sting technique to grow crystals in space. Authors’ experiences of conducting two
space shuttle experiments relating to solution crystal growth are also detailed in this work.
The complexity of these solution growth experiments to grow crystals in space are discussed.
These happen to be some of the early experiments performed in space, and various lessons
learnt are described. A brief discussion of protein crystal growth that also shares basic
principles of solution growth technique is given along with some flight hardware information
for its growth in microgravity.
Key Words: Solution crystal growth; Microgravity; Triglycine sulfate; Protein crystals,
Spacelab-3, International Microgravity Laboratory-1

+ Present Address: NASA Administrator’s Fellow, EV-43 ISHM and Sensors Branch, NASA
/Marshall Space Flight Center, Huntsville, AL 35812, USA
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Contents
1.0 INTRODUCTION
2.0 CRYSTALLIZATION: NUCLEATION AND GROWTH KINETICS
2.1 Expression for supersaturation
2.2 Effects of convection in solution growth
2.2.1 Natural convection
2.2.2 Forced convection
2.3 Effect of impurities
3.0 C LASSIFICATION OF CRYSTAL GROWTH
4.0 LOW TEMPERATURE SOLUTION GROWTH
4.1 Solution growth methods
4.1.1 Slow cooling method
4.1.2 Slow evaporation method
4.1.3 Temperature gradient method
4.1.4 Chemical/Gel method
5.0 SOLUTION GROWTH BY TEMPERATURE LOWERING
5.1 Solvent selection and solubility
5.1.1 Solubility determination
5.2 Design of a crystallizer
5.2.1 A Typical solution crystal growth crystallizer
5.2.2 Crystal seed-holder

5.2.3 Preparation of seed crystal and mounting

5.3 Solution preparation and starting a growth run
6.0 TRIGLYCINE SULFATE CRYSTAL GROWTH– A CASE STUDY
6.1 Growth of single crystals of triglycine sulfate
6.2. Growth kinetics and habit modification
6.2.1 Effect of seed crystal
6.2.2 Effect of growth temperature and supersaturation
6.2.3 Effect of pH of the solution
6.2.4 Effect of impurities
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7.0 SOLUTION GROWTH OF TRIGLYCINE SULFATE CRYSTALS IN
MICROGRAVITY ABOARD SPACELAB-3 AND IML-1
7.1 Rationale for solution crystal growth in space
7.2 Crystal growth method in space
7.2.1 Cooled sting technique
7.2.2 Flight hardware
7.2.3 Flight optical system
7.3 Results and Discussion
8.0 PROTEIN CRYSTAL GROWTH
8.1 Protein crystal growth methods
8.2 Protein crystal growth mechanisms
8.3 Protein crystal growth in microgravity
9.0 CONCLUDING REMARKS
Acknowledgements

References

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1.0 INTRODUCTION
The growth of crystals with tailored physical and chemical properties,
characterization of crystals with advanced instrumentation and their eventual conversion into
devices, play a vital role in science and technology. Crystal growth is an important field of
materials science, which involves controlled phase transformation. Growth of crystals
from solution at low temperature is one of the important techniques in the field of
science: pharmaceutical, agriculture and materials science. Crystal growth acts as a
bridge between science and technology for practical applications. In the past few decades,
there has been a growing interest in the crystal growth process, particularly in view of the
increasing demand for materials for technological applications. The strong influence of
single crystals in the present day technology is evident from the recent advancements in the
fields of semiconductors, transducers, infrared detectors, ultrasonic amplifiers, ferrites,
magnetic garnets, solid state lasers, nonlinear optic, piezoelectric, acousto-optic,
photosensitive materials and crystalline thin films for microelectronics and computer
applications. All these developments could be only achieved due to the availability of single
crystals such as silicon, germanium, gallium arsenide, and also with the discovery of
nonlinear optical properties in some inorganic, semi-organic and organic crystals. Researchers
have always been in the search of new materials for the growth of single crystals for new
applications and modifying present crystals for various applications. Any crystal growth
process is complex; it depends on many parameters which can interact. A complete
description of a process may well be impossible, since it would require the specifications of
too many variables. That is why, sometimes crystal growth is called art and science but like
other crafts, it can provide great satisfaction after a successful crystal growth of a desired
material.
The solid-state materials can be classified into single crystals,
polycrystalline, and amorphous materials depending upon the arrangement of constituent
molecules, atoms or ions. An ideal crystal is one, in which the surroundings of any atom
would be exactly the same as the surroundings of every similar atom in three dimensions.
Real crystals are finite and contain defects. The consistency of the characteristics of devices
fabricated from a crystal depends on the homogeneity and defect contents of the crystals.
Hence, the process of producing single crystals, which offer homogeneous media in the
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atomic level with directional properties, attracts more attention than any other process. The
methods of growing crystals are mainly dictated by the characteristics of the material
and the desired size of the crystal. The method of growing crystals at low and high
temperature can be broadly divided into the following six categories: (i) Growth from
aqueous solution (low temperature growth); (ii) Growth by gel method (low temperature
growth); (iii) Growth from flux or top seeded solution growth method (high temperature
growth); (iv) Hydrothermal growth (high temperature growth); (v) High pressure growth
(high temperature growth); and (vi) Growth by electrodeposition
Growth of bulk crystals from aqueous solution is technically very important. Besides
bulk crystal growth, this method is also used for the purification of materials and the
separation of impurities. Growth of large single crystals from aqueous solution is of interest
for essentially two reasons. First, there is a growing need for solution-grown crystals in the
area of high-power laser technology like potassium dihydrogen phosphate (KDP) type
crystals. Second, research on this area of crystal growth and the corresponding in-depth
examination of several key parameters provides fundamental case studies generating theory
and technology, applicable to all of solution crystal growth processes, including new aqueous
growth systems and high temperature solution growth.
In this chapter, the fundamental aspects of solution growth and the different methods of
bulk crystal growth from solution are described along with solution crystal growth in the
microgravity environment of space. Based on extensive experience of the authors in growing
inorganic and organic crystals on earth and in space, the authors have tried to give a lucid
explanation of the fundamentals of solution crystal growth and crystal growth systems. However,
enough details are given on fabrication of crystallizers, associated instruments, and techniques so
that new researcher may be able to design and set up his/her own solution crystal growth system
after review of this chapter. Furthermore, growth and perfection of technologically important
crystal from aqueous solution based on a case study of triglycine sulfate is presented. Effects of
various parameters such as design of the seed holder, seed morphology, characteristics of the
solution such as pH, temperature of growth, dopants, impurities; and microgravity on the physical
properties are presented in detail.

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2.0 CRYSTALLIZATION: NUCLEATION AND GROWTH KINETICS
The study and investigation of crystal growth implies the determination of growth
laws, growth mechanisms and explanation of final result, i.e. the crystal habit. These aspects
are interconnected. Since the growth rate of a face depends on its growth mechanisms and
contributes to define the crystal habit, the detailed knowledge of these aspects is essential for
the production of crystals of specific physical or morphological properties. The crystal
growth is due to deposition of solute particles on the crystal faces, which can grow layer by
layer at different rates. The growth rate of a face, i.e. advancement of its surface in the
normal direction per unit time, depends upon internal and external factors. Internal factors
are the surface structure of faces, which in turn are related to the bulk crystal structure, and
their degree of perfection. Defects usually occur in the crystals and can emerge at the surface,
affecting the growth kinetics. External factors are supersaturation, solute concentration which
is related to solubility, temperature of the solution, solution composition, mechanical
conditions such as still or stirred solution, presence of impurities, magnetic field, and
gravitational field. The crystal growth of a face is a succession of complex processes, which
take place at the interface between the liquid and solid phase. It therefore implies transport of
matter and energy across the interface, which is the site of major importance in crystal
growth.
In the following section, the fundamentals of nucleation and crystal growth at low
temperature solution are described.
2.1 Expression for supersaturation
The supersaturation of a system can be expressed in a number of ways. A basic unit of
concentration as well as temperature must be specified. The concentration driving force (ΔC),
the supersaturation ratio (S) and relative supersaturation (σ) are related to each other as
follows:
The concentration driving force
ΔC = C - C* (1)
where C is the actual concentration of the solution and C* is the equilibrium concentration at
a given temperature.
Supersaturation ratio
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S = C / C* (2)
Relative supersaturation,
σ = (C - C*) / C* or σ = S - 1 (3)
If the concentration of a solution can be measured at a given temperature and the corresponding
equilibrium saturation concentration is known, then the supersaturation can be estimated.
The required supersaturation can be achieved either by cooling/evaporation or addition
of a precipitant. Meirs and Isaac reported a detailed investigation on the relationship between
supersaturation and spontaneous crystallization [1]. The results of their analysis are shown in
Fig. 1. It shows three zones, which are termed as region I, II and III. The lower continuous
line is the normal solubility of the salt concerned. Temperature and concentration, at which
spontaneous crystallization occurs, are represented by the upper broken curve, generally
referred as the super-solubility curve. This curve is not well defined as the solubility curve
and its position in the diagram depends on the degree of agitation of the solution. The three
zones are defined as follows:
I. The stable (undersaturated) zone, where crystallization is not possible
II. The metastable zone, where spontaneous crystallization is improbable. However,
if a seed crystal is placed in such a metastable solution, growth will occur
III. The unstable or labile (supersaturation) zone, where spontaneous crystallization
is more probable.
The achievement of supersaturation is not sufficient to initiate the crystallization. The
formation of embryos or nuclei with a number of minute solid particles present in the
solution, often termed as centers of crystallization, is a prerequisite. Nucleation may occur
spontaneously or it may be induced artificially. Broadly nucleation can be classified into
primary and secondary. All types of nucleation, homogeneous or heterogeneous in systems,
which do not contain crystalline matter comes under primary. On the other hand, nucleation
generated in the vicinity of crystals present in a supersaturated system is termed as
secondary.

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Fig. 1 Meirs and Issac solubility curve
The formation of stable nuclei occurs only by the addition of molecule (A ), till a
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critical cluster is formed.
A + A → A (critical cluster) (4)
n-1 1 n
Subsequent additions to the critical cluster result in nucleation followed by
growth. The growth units (ions or molecules) in a solution can interact with one
another resulting in a short-lived cluster. Short chains may be formed initially or flat
monolayers and eventually the lattice structure is built up. This process occurs very rapidly
and continues in regions of very high supersaturation. Many nuclei fail to achieve
maturity and simply dissolve due to their unstable nature. If the nuclei grow beyond a
certain critical size, they become unstable under the average conditions of supersaturation in
the bulk of the solution. The formation of a solid particle within a homogeneous solution
results from the expenditure of a certain quantity of energy.
The total quantity of work `W' required for the formation of a stable nucleus is equal
to the sum of the work required to form the surface W (a positive quantity) and the work
S
required to form the bulk of the particle Wv (a negative quantity).
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W = Ws + Wv (5)
The change in Gibbs free energy (ΔG) between the crystalline phase and the
surrounding mother liquor results in a driving force, which stimulates
crystallization. This ΔG is the sum of surface free energy and volume free
energy.
ΔG = ΔGs + ΔGv, (6)
For a spherical nucleus
ΔG = 4π r2γ + 4/3 π r2ΔGv (7)
where r is the radius of nucleus, γ is the interfacial tension and ΔGv is the free energy
change per unit volume.
For rapid crystallization, ΔG < 0; the first term in the above equation expresses
the formation of new surface and the second term expresses the difference in
chemical potential between the crystalline phase (µ) and the surrounding mother liquor
(µ ). At the critical condition, the free energy formation obeys the condition dΔG/dr =
o
0. Hence the radius of the critical nucleus is expressed as
r* = 2 γ/ ΔGv (8)
The critical free energy barrier
ΔG* = (16 π γ3 v2)/3(Δμ)2 (9)
The number of molecules in the critical nucleus is given as
I* =4/3 π γ (r*)3 (10)
The crucial parameter between a growing crystal and the surrounding mother liquor is the
interfacial tension (γ). This complex parameter can be determined by conducting the nucleation
experiments.
Growth of crystals from the vapor, melt or solution occurs only when the medium is
supersaturated. The process involves at least two stages [2]:
(1) formation of stable three-dimensional (3D) nuclei and
(2) development of the stable 3D nuclei into crystals with well-developed faces.
The formation of 3D nuclei is usually discussed in terms of reduction in the Gibbs free
energy of the system. At a given supersaturation and temperature, there is a critical value of the
free energy at which 3D nuclei of a critical radius are formed. Only those nuclei which are
greater than the critically-sized nucleus are capable of growing into crystals of visible size by
the attachment of growth species (i.e. molecules, atoms or ions) at energetically favorable
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growth sites like kinks (K) in the ledges (L) of a surface. The surfaces of growing crystals may
be flat (F), stepped (S) or kinked (K). However, crystals of visible size are usually bounded
by the slowly-growing F faces which grow by the attachment of growth units at energetically
favorable sites. Fig. 2 shows different positions for the attachment of growth units at a flat
crystal-medium interface of a simple cubic lattice. A growth unit attached at the surface
terrace (T) , a smooth ledge (L) and a kink site (K) has 1, 2 and 3 out of the 6 nearest
neighbors, respectively. Therefore, a growth unit arriving on the surface terrace, at the terrace
ledge and at the kink simply loses one, two and three degrees of freedom. If φis the binding

energy per pair, the corresponding binding energy of a growth unit attached at these sites is φ,
2 φ and 3 φ, respectively. Since the probability of capture of a growth unit at a given site depends
through terms exp(n φ /kT) (where n is the number of bonds formed, k is the Boltzmann
constant and T is the temperature in Kelvin), the growth unit has a much higher probability

Fig. 2. Different positions for the attachment of growth units at a flat
crystal-medium interface of a simple cubic lattice.
of becoming a part of the crystal at the kink site rather than at the ledge or at the surface
terrace. Consequently, in contrast to ledges, the contribution of kinks is overwhelmingly high in
the rate v of displacement of a step along the surface and in the rate R of displacement of the
1 0